1. Field of the Invention
This invention relates to a photosensitive composition containing at least one fluorinated multifunctional acrylate or methacrylate, collectively referred to herein as acrylate, and a waveguide device fabricated with the said composition. This acrylate composition demonstrates excellent coating, fast cure, high photo contrast, low optical absorption loss in the telecommunication wavelength region of 1300–1610 nm, low birefringence, high thermo-optic (TO) coefficient and high thermal stability. The waveguide device is useful for fiber optic telecommunication networks, which use single mode optical waveguides to interconnect various fiber optic devices as well as optical fibers.
2. Description of the Prior Art
A waveguide is a planar structure comprising a high refractive index material in the core and low refractive index materials in the cladding that surrounds the core. The core geometry and the refractive index difference between the core and the cladding determine the waveguide's optical characteristics such as its mode size. The waveguide can be in any form or shape depending on its end use. The waveguides used for telecommunications applications can be used to produce various components intended to interface with fiber-optic networks such as thermo-optic switches, splitters, combiners, couplers, filters, tunable filters, attenuators, wavelength cross connects, channel monitors and add-drop multiplexers. The fiber optic applications require that the materials and their waveguide devices meet many specifications such as low optical insertion loss in the useful wavelength region of 1300–1610 nm, high TO coefficient (|dn/dT|), low birefringence and high reliability. This is challenging and requires a unique material and device approach to meet all the requirements.
It is known in the art that a photosensitive composition can be patterned by an actinic radiation such as UV light, e-beam and X-ray to produce waveguides. One method used to form waveguides involves the application of standard photolithographic processes. Using the photolithographic process, an image is projected from a mask onto a photosensitive layer carried upon a substrate. Among the many known photosensitive polymers, acrylate materials have been widely studied as waveguide materials because of their optical clarity and low birefringence. The details of the prior art are described in U.S. Pat. Nos. 4,609,252; 4,877,717; 5,054,872; 5,136,682; 5,396,350; 5,402,514; 5,462,700; 5,481,385; 6,023,545 and 6,114,090, which are incorporated herein by reference.
Although the prior art teaches how to fabricate waveguides with photopolymers, practicing of the art has not led to devices that meet all the requirements for practical use in telecommunication networks. Typical prior art devices use hydrocarbon materials that have very high absorption loss in the spectral region from 1,300 to 1,610 nm. Also prior art devices use materials that have high shrinkage upon curing, leading to high residual stress and hence high scattering loss. All these losses lead to devices that have unacceptably high insertion loss.
To achieve low insertion loss it is necessary to simultaneously realize low absorption and scattering losses of the waveguides in the telecommunication wavelength region of 1,300–1,610 nm and low coupling loss between the waveguides and their pigtailed fibers. To realize low absorption loss it is required to use materials that have low absorption at 1,300–1,610 nm. To realize low scattering loss it is required to use materials and device fabrication processes that allow for homogeneity and minimize stress. To realize low coupling loss it is required that waveguides match optical fibers in both cross section and mode size. To realize the matched cross section and mode size, high contrast materials are utilized to control the waveguide size as well as the capability to control refractive indexes of the materials.
Prior art references such as U.S. Pat. Nos. 3,779,627; 4,138,194; 5,062,680; 5,822,489 and 6,005,137, which are herein incorporated by reference, have taught that replacing hydrogen-carbon bonds with fluorine-carbon bonds or deuterium-carbon bonds will reduce the absorption loss of an organic material in the near IR wavelength region of 1,300–1,610 nm. Replacing hydrogen with fluorine in a material also decreases the refractive index of the material.
Some fluorinated acrylate compositions disclosed in the prior art contain low molecular weight or mono-functional acrylates. There are several drawbacks to such materials. First of all, the monomers do not have the minimum viscosity required to form a uniform coating with sufficient thickness. Second, the high volatility of low molecular weight monomers impairs production of waveguides and other coatings. The highly volatile monomers not only contaminate the curing chamber but also make it extremely difficult to achieve consistent material properties, including refractive index, after curing. Third, the low molecular weight of the monomers leads to very high shrinkage (up to 20%) upon curing. This high shrinkage causes high residual stress. Fourth, it is difficult to fully cure mono-functional monomers with UV light. The residual monomer will cause reliability and environmental problems.
The urethane linkage has been used to extend the molecular chain of fluorinated acrylates to increase molecular weight and viscosity. This is intended to overcome the abovementioned problems. However, because of the presence of the N—H bond, a urethane group absorbs moisture and has strong absorption from 1300 to 1600 nm. Hence the urethane group is unsuitable for applications requiring low optical loss at near IR wavelengths. Likewise, the urethane group is inappropriate for applications requiring moisture resistance.
In addition to the above-mentioned requirements, thermo-optic (TO) devices such as TO switches, variable optical attenuators and tunable filters require high TO coefficient (|dn/dT|) to reduce power consumption and increase tuning range. The TO coefficient (|dn/dT|) is the change in refractive index, n, of a material induced by a change in temperature, T. For tunable grating devices a more useful term, dλ/dT, which defines the tuning efficiency of a grating, is used. For a typical organic polymer the change in refractive index with temperature in the 1300–1610 nm wavelength region is mainly caused by its density change with temperature. When the polymer material is heated it expands and its density decreases, leading to the decrease of its refractive index. Consequently dn/dT is negative.
Low birefringence is also important to minimize signal distortion and polarization dependent loss (PDL) of a waveguide device. It is known in the art that one way to reduce birefringence is to use an isotropic material with low stress and strain optic coefficients.
Therefore there is a need for an acrylate composition having high viscosity, low volatility, low absorption in the wavelength region from 1300 to 1610 nm, high TO coefficient, low birefringence and low shrinkage upon curing.